Open the editorEditor
Built-in project · Arduino Uno R3

Photoresistor night light: A0 dims the LED on pin 9, on an Arduino Uno

A photoresistor and a 10k resistor in a divider on A0 set the brightness of an LED on pin 9: the darker the room, the brighter the LED. The whole build, an Arduino Uno and 4 more parts, runs here in your browser on the firmware below; open it in the editor to change the wiring or the code and run it again.

Open in the simulatorRead the code

Teaching? Assign it to your class in one click.

Beginner About 10 minutes to build and run. Runs in your browser. Free, and no account needed.

The parts retail for about $30. On Mokxi they're free.

The circuit runs here once the page has loaded.

What you'll learn

  • A light sensor in a voltage divider
  • Reading light with analogRead
  • Turning a reading into brightness with map
  • Why the light and LED respond in opposite directions
  • Difficulty: Beginner
  • Time: About 10 minutes
  • Board: Arduino Uno R3
  • Parts: 6

How it works

The photoresistor and a 10k resistor form a voltage divider on A0. The 10k goes to 5V and the photoresistor goes to ground. In bright light the photoresistor has low resistance and pulls A0 down toward 0 V. In the dark its resistance is high, so the 10k pulls A0 up toward 5 V.

The sketch reads A0, scales 0 to 1023 into 0 to 255, and sets the brightness of the LED on pin 9. So the darker it gets, the brighter the LED. It also prints every reading to the serial monitor.

Parts list

6 parts, plus the jumper wires. They retail for about $30; on Mokxi they're free.

Kits that can build it

These 10 starter kits have every part in the circuit. Each kit's page lists what else it can build.

Wiring table

Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.

Part and pinConnects to
Light sensor pin 1Arduino Uno R3 pin GND
LED, yellow pin CArduino Uno R3 pin GND
Resistor, 220 Ω pin 1Arduino Uno R3 pin 9
Resistor, 10k Ω pin 2Arduino Uno R3 pin 5V
Light sensor pin 2Arduino Uno R3 pin A0
Resistor, 10k Ω pin 1Arduino Uno R3 pin A0
LED, yellow pin AResistor, 220 Ω pin 2

The code

The firmware exactly as the editor opens it. Change a line there and press Run: it compiles in the browser.

sketch.ino · Arduino Uno R3
// A night light: the darker it gets, the brighter the LED.
//
//   10k from 5V -> A0; the photoresistor from A0 -> GND
//   pin 9 -> 220 ohm -> LED -> GND
//
// In bright light the photoresistor's resistance is low, so it pulls A0
// down towards 0 V; in the dark it is high, and the 10k pulls A0 up towards
// 5 V. map() turns the 0 to 1023 reading into a 0 to 255 brightness.
//
// Drag the light level on the photoresistor.

const int SENSOR = A0;
const int LED_PIN = 9;

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: night light, photoresistor on A0, LED on pin 9");
}

void loop() {
  int reading = analogRead(SENSOR);
  analogWrite(LED_PIN, map(reading, 0, 1023, 0, 255));
  Serial.println(reading);
  delay(100);
}

Code walkthrough

The key functions of the sketch, and what each one does.

setup sketch.ino

Sets LED_PIN (pin 9) as an output and starts serial at 9600 with a short message.

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: night light, photoresistor on A0, LED on pin 9");
}

loop sketch.ino

Reads SENSOR (A0), maps the reading from 0 to 1023 into 0 to 255, and passes it to analogWrite. Dark means a high reading and a bright LED. It prints the raw reading and waits 100 ms.

void loop() {
  int reading = analogRead(SENSOR);
  analogWrite(LED_PIN, map(reading, 0, 1023, 0, 255));
  Serial.println(reading);
  delay(100);
}

Troubleshooting

The usual mistakes with this circuit, and the fix for each.

  • The LED gets brighter in the light, not the dark This project
    The divider is swapped. The 10k should go to 5V and the photoresistor to ground, with A0 between them.
  • The LED never changes This project
    Check that A0 connects to the point between the photoresistor and the 10k resistor.
  • The reading barely changes, or sits stuck near 0 or the top of the range. Light sensor
    An LDR is a resistor, so use it in a divider with a fixed resistor, about 10k, and read the middle node. Without the divider the analog pin sees nothing useful.
  • Readings go the wrong way: higher in the dark. Light sensor
    Which side the LDR sits on decides the direction. LDR on the supply side gives a higher reading in the light. LDR on the ground side gives a lower reading in the light. Swap them if needed.
  • The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
    Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure.
  • A pull-up or pull-down resistor is missing, so the input floats and reads random values. Resistor
    Tie the input to 5 V or ground through about 10k. Or use INPUT_PULLUP and wire the switch to ground.
  • The LED does not light, or it lights once and then dies. LED
    LEDs have polarity. The longer leg (anode) goes toward the positive side. Always add a series resistor, about 220 ohms to 1 k ohm on a 5 V pin.
  • A blue or white LED stays dark on 3.3 V. LED
    Blue and white LEDs drop about 3 V, so 3.3 V leaves almost nothing for the resistor. Use a 5 V supply, or a smaller resistor and a red or green LED.

Change it and keep it

Open it in the editor, change the circuit or the code, and keep your version in a free account.

Start building free
Open it in the editor